1. Field of the Invention
The present invention relates to displacement sensing apparatus and in particular optical displacement transducers.
2. The Prior Art
Optical displacement transducers are known in which a coded track is moved between a source of light and a photo detector so that the displacement, or the position of the track relative to the transducer, can be determined. In a simple form of such transducer, the track can comprise an array of identical marks along its length and movement along the track is detected by counting the marks passing a reading head. This arrangement requires the transducer to keep a continuous count of the marks in order to provide an indication of the position along the track. Such a system is vulnerable in the event of a power failure resulting in loss of the latest count value.
Alternative arrangements which are known employ tracks from which a binary word can be read at any position along the track, with the word uniquely defining the position along the length of the track. The binary word representing the position along the track may be read by reading separate bit indications across the width of the track, for example where the track is of the form commonly used in shaft encoders. In an alternative example the track may be formed as a pseudo random binary sequence (PRBS) whereupon the unique binary word comprises a predetermined minimum number of adjacent bits along the length of the track. Examples of this latter arrangement are contained in GB-A-No.2126444 (corresponding to U.S. Ser. No. 606,814).
In either of the above cases, it has hitherto been necessary to provide some electronic circuitry at the transducer with appropriate power supply, in order to develop at the transducer electrical signals representing the displacement of position read by the transducer. As a result, where, as in most cases, the displacement value provided by the transducer is required at an instrument or apparatus somewhat distant from the transducer, the electrical signals indicating the displacement must be transmitted by means of electrical wires. Furthermore, the electrical power to the transducer must be supplied along wires.
In some applications, it is desirable to provide transducers for measuring various parameters, including for example displacement, which contain no electronic components and which include no electrical interconnections between the transducer and remote apparatus in which the measured parameter is to be utilised. It has been proposed to employ transducers which can be connected to the remote utilisation apparatus by means of optical fibres. However, there has as yet been no satisfactory proposal for employing solely optical fibre interconnections between utilisation apparatus and an optical displacement transducer of the kind described above in which a binary word is read from the track to identify the position along the length of the track.